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Update on testing the thermistor circuit board.

The final version of the circuit.

The final version of the circuit.

All the troubleshooting is complete and the circuit is working well. They say experience is the best teacher… just too bad it comes too late. My dad has a related saying: “Experience is the best teacher, but it is a heck of a lot easier if it is the other guy’s experience.” So, I will describe the stupid things I do as well as the victories. To paraphrase Edison making his light blub. “Well that didn’t work. Time to try something different.”

When I ended the last post, I had 4 of the 6 circuits working and I was in the process of removing the op-amp for circuit no. 1.  STUPID!   On the way home from work I figured out the problem, but by then I had to replace the I.C.   The first stage in the amplifier worked well, it was the 2nd stage that did not.  The 2nd stage was saturating and when I measured the voltage on the inverting input of the 2nd stage it was displaying exactly the same voltage as the output of the 1st stage.  My first assumption was the feed back resistor to the 2nd stage was bad or I had a bad connection.  What it turned out to be was I had R6 adjusted to zero ohms.   This made the gain at infinity and “washed out” any feedback voltage.   DUH!  Once I replaced the I.C. and adjusted R6 to mid-range the pre-calibration went smoothly.

If I redesign the circuit I will look at replacing R6 with a 5K resistor and putting a 4.7K or 5.1K in series.  That will require another computer run to see if I have the necessary range after tolerance build up from all the components.

The final circuit, circuit no. 2 was a little more difficult.  That was actually the first one I had built and I was a still in the process of learning how to do things for this point-to-point wiring method.  I had several bad connections.  Once I worked through all of those it was not a big problem.  Should you decide to build a circuit using this method, the best recommendations I can make is to use the resistor and other components wire where possible to avoid having to strip the wire-wrap wire.  This has the advantage of holding the wire in place while you use one hand to control the solder and the other hand to hold the soldering iron.   It is impossible to use the component wires often times because of the location of the connections and insulated wire must be used.  In that case, the 2nd recommendation is to cut and strip both ends of the wire wrap wire before soldering either end.  I tried to solder one end and then strip the other end afterwards and ended up very frustrated.

The calibration test chamber.

The calibration test chamber.

The next step was to move on and start the calibration of the actual thermistors.  To do that I needed a insulated box to obtain 158 deg F (70 deg C) temperature and also 25 deg C (78 deg F).  Our house temperature is normally about 72 deg F so I needed to add a little heat in the box for 78 deg F.  Inside the Styrofoam cooler I installed a lamp and a fan.   The fan keeps the air moving so there are no temperature differences in the box.

Inside of the test chamber.

Inside of the test chamber.

The fan is something I removed from some junk electronics.   The light bulb is a 60 watt incandescent lamp.  The pan of water was just to add some thermal mass so the temperature would remain constant while I adjusted the potentiometers.  Usually the pan is covered to keep evaporation at a minimum. This worked well for the 78 deg temp and I have calibrated R4 for the actual thermistors.  Also, because the thermistors are the component with the greatest tolerance, I have assigned each one to a circuit before calibration.

The mounting of the reference thermometer and the thermistors under test.

The mounting of the reference thermometer and the thermistors under test.

The thermistors were installed in the chamber at approximately the same depth as the reference thermometer.  This was to make the everything as consistent as possible.

My big screw up so far is that I attempted to prop-up the light with a little frame I had made from popsicle sticks.  (I still have a lot of those laying around.)  I used hot melt glue because it was quick and I assumed it was good for the temperature.   It wasn’t, and when I got near the 178 degree range the frame fell apart.     I have since replaced that frame with the good old trusty bent coat hanger.  However, that was too late before I needed to get this post out.

The Arduino readings of the channels.

The Arduino readings of the channels.

The final work I got accomplished was to get the arduino up and running reading the output from the thermistor board.   The final calibration will include the Arduino A/D converter.  I was able to get it to display all six channels and the readings seem to be stable and made sense although the board is not completely calibrated yet.

All in all a lot was accomplished,but still very far to go.

Gary


 

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